ERA20LA341

The fatigue failure of the No. 1 cylinder exhaust valve spring retainer due to air trapped in the lubrication system, which resulted in a total loss of engine power.

While in cruise flight at the conclusion of a flight lesson, the airplane suddenly began to vibrate severely. Despite the flight instructor’s efforts, the Rotax 900 series engine eventually lost total power, and he performed a forced landing to a soybean field. During the landing, as the airplane slowed, the nose landing gear dug into the soft earth and separated, and the airplane nosed over. Examination of the engine revealed that the No. 1 cylinder’s exhaust valve spring retainer was broken, and the exhaust valve had fallen into the No. 1 combustion chamber, which resulted in the loss of power. This valve spring retainer failure was not the first one with a Rotax 900 series engine; in 2017, at the end of a cross country flight, an airplane powered with the same series engine experienced a total loss of engine power, and the pilot performed a forced landing during which the airplane sustained substantial damage. Examination of that engine revealed the presence of a broken valve spring retainer that had resulted in the loss of power. Additionally, it was discovered that the valve spring retainer displayed evidence of metal fatigue. During 2019 and 2020, in addition to this accident, three more cases of broken valve spring retainers on the Rotax 900 engine series occurred in the United States. All the engines had differing hours of operation. Extensive metallurgical examination of the engine components from these four engines revealed that they met their specifications, and the fractured surfaces on the valve spring retainers revealed the presence of fatigue with pronounced vibration stripes, which was the same pattern observed on the valve spring retainer from the 2017 accident. Review of the engine manufacturer’s published guidance revealed that air could be introduced into the oil lubrication system through several means, including exceedance of the maximum bank angle limitation of 40º, poorly or insufficiently vented hydraulic valve tappets, lack of proper oil system purging, spinning the propeller in the reverse direction from normal rotation, or opening portions of the oil system during maintenance or servicing. Testing an exemplar engine with air introduced into the lubrication system revealed that with air trapped in the hydraulic tappets, it took about 6.5 minutes of engine operation at 2,538 rpm for air to be purged from the tappets allowing them to work as designed. This indicated that with air trapped in the hydraulic tappets, the valve train could be overloaded, which could lead to a fatigue crack and breakage of a valve spring retainer; this was likely the reason for the fatigue cracking of the valve spring retainers in the 2017 accident, in this accident, and in the other four 2019-2020 engine failures. During this investigation, the engine manufacturer reviewed its records and found a total of 18 production engine failures due to broken valve spring retainers. The engines were installed on multiple types of aircraft with a large spread in operating hours from as low as 7 hours to as high as 1,936.6 hours. All the components examined met their specifications, and not all the engines were affected by service bulletins that had been issued due to deviations in the manufacturing process of the valve push-rod assembly, which could result in partial wear on the rocker arm ball socket and initiate rocker arm cracking leading to a malfunction of the valve train. These engine failures indicated that valve train failure could occur for reasons other than the push-rod manufacturing issue such as air being introduced into the lubrication system. Additionally, after the engine manufacturer’s record review, an engine in an airplane that was produced in 2021, which should have had all changes included in Rotax guidance materials incorporated before it was placed into service, experienced a valve spring retainer failure, confirming that valve train failure could occur for reasons such as air being introduced into the lubrication system.

HISTORY OF FLIGHT On September 28, 2020, about 1800 eastern daylight time, a Tecnam P92 airplane, N562TU, was substantially damaged when it was involved in an accident near Centerville, Maryland. The flight instructor and student pilot were not injured. The airplane was operated as a Title 14 Code of Federal Regulations (CFR) Part 91 instructional flight. The airplane was being operated by Chesapeake Sport Pilot, a 14 CFR Part 91 flight school based at Bay Bridge Airport (W29), Stevensville, Maryland. According to the flight instructor, he and a student pilot were returning to W29 when the airplane suddenly began to vibrate severely. This was followed by a reduction in engine rpm from about 5,100 rpm to 4,500 rpm. Review of onboard recorded data indicated that the fuel pressure, cylinder head temperature, and oil temperature remained relatively steady until the loss of power occurred. The flight instructor took over the flight controls from the student pilot and manipulated the throttle control to see if a different power setting would reduce the vibrations. Movement of the throttle control did not elicit a corresponding response from the engine. The flight instructor then ensured that the fuel valves were all on and turned on the electric fuel pump. There were no changes to the vibrations or power. Assessing the situation as an impending engine failure, the flight instructor configured the airplane for best glide speed, turned towards the nearest field for a potential forced landing, and made a “Mayday” transmission. About 1 minute later, the engine lost total power. He attempted to restart the engine, but the engine would not crank when the starter was engaged, and all the avionics in the airplane shutdown. About 30 seconds later, all the displays came back on, and the flight instructor configured the airplane for landing and touched down uneventfully in the soybean field he had selected. As the airplane slowed, the airplane’s nose dropped to the ground, and the nose landing gear dug into the soft earth. The nose landing gear separated, and the airplane nosed over. AIRPLANE INFORMATION The strut-braced, high-wing, two-seat, airplane was made of sheet and tubular aluminum. The design complied with Federation Aeronautique Internationale microlight rules and Federal Aviation Administration (FAA) light sport aircraft rules. It was equipped with an American Society for Testing and Materials compliant, 4-cylinder, horizontally-opposed, 100-horsepower, Rotax 912 ULS 2 engine. The engine used a single central camshaft with hydraulic tappets. The cylinder heads were liquid cooled, and the cylinders were ram air cooled. The oil system was a dry sump, forced lubrication system. The engine used a reduction gearbox to drive the two-bladed, fixed-pitch Sensenich propeller. According to FAA and airplane maintenance records, the airplane was manufactured in 2017. The airplane's most recent condition inspection was completed on March 3, 2020. At the time of the inspection, the airplane had accrued about 1,081 hours of operation, and the engine had accrued about 734 hours of operation. FLIGHT RECORDERS The airplane was not equipped with a flight data recorder nor was it required to be under CFR Part 91. It was equipped with two Garmin G3X flight displays that recorded historical information at a variable rate of about 10 Hertz to internal non-volatile memory. Review of the data revealed a noticeable gap in the data toward the end of the flight. This was indicative of the power interruption to the displays as described by the flight instructor and resulted in the displays writing the remaining flight data to a separate file. This process resulted in a gap in the recorded data. WRECKAGE AND IMPACT INFORMATION Postaccident examination revealed that the airplane had sustained substantial damage to the fuselage and both wings. Examination of the engine, serial number (S/N) 9569181, revealed that there were no anomalies with the oil system and that there was oil throughout the engine. Aluminum debris and engine oil were found in the carburetor for Nos. 1 and 3 cylinders. The No. 1 cylinder’s exhaust valve spring retainer was broken, and the exhaust valve had fallen into the cylinder’s combustion chamber. A buildup of metallic material was found in the No. 1 intake manifold. The stem of the No. 1 exhaust valve was in place; however, the head of the exhaust valve was no longer attached. After removal of the No. 1 cylinder head, damage to the cylinder head, piston, and valves was discovered. The No. 1 exhaust valve head was found imbedded in the No. 1 intake valve. The No. 1 piston had a large hole in the crown of the piston; the No. 1 cylinder displayed damage; and the No. 1 connecting rod was bent and twisted. Additionally, the No. 3 cylinder head was removed, and metallic material was found in the No. 3 combustion chamber. The No. 3 piston displayed damage, and the No. 3 cylinder was damaged and displayed multiple impact marks. TESTS AND RESEARCH Accident with N561TU The National Transportation Safety Board (NTSB) first became aware of valve spring retainer fracturing issues with Rotax 900 series engines in 2017 due to an accident that occurred in Stevensville, Maryland, with another Tecnam P92 airplane, N561TU, that was also operated by Chesapeake Sport Pilot. (NTSB Case No. ERA17LA246). The airplane was powered by a Rotax 912 ULS2-01 engine, S/N 9569084. In this accident, the airplane experienced a total loss of engine power at the end of a cross country flight, and the pilot performed a forced landing during which the airplane sustained substantial damage. The airplane had recently been purchased, and the engine had 13.2 hours total operating time. Review of onboard data indicated that the fuel pressure, cylinder head temperature, and oil temperature remained relatively steady until the loss of power occurred, which indicated that the engine failure likely did not involve the fuel system, cooling system, or lubrication system. Examination of the engine revealed that there was no oil in the oil line between the oil thermostat and oil pump. The oil pump drive pin also displayed excessive wear in relation to the operating hours of the engine, and the magnetic plug was covered in metallic particles, although the oil filter was clean. Further examination of the engine revealed that the No. 1 cylinder was damaged, and evidence of bluing was present. The cylinder’s exhaust valve spring retainer was fractured in half, and one half of the cotter was fractured. A small ridge could be felt on the exhaust valve spring retainer and galling (a rough surface) was visible on the exhaust valve bore in the cylinder head. Examination of the fractured surface on the exhaust valve spring retainer revealed the presence of fatigue with pronounced vibration stripes when viewed with an electron microscope; however, the heat treatment corresponded to the target specifications, as did the statistical process control value. According to the NTSB’s final report on the accident, the root cause of the failure could not be determined based on the available information. Additional Valve Spring Retainer Fractures In 2019 and 2020, another four valve spring retainer fractures occurred in the United States involving the following aircraft: N1PJ, N204BF (NTSB Case No. WPR20LA012), N117BF, and N562TU (this case). Examinations of the damaged engines revealed: o S/N 4421750 (N1PJ), intake valve failure, broken valve spring retainer cylinder No. 2 o S/N 9569290 (N204BF), intake valve failure, broken valve spring retainer, cylinder No. 2 o S/N 9569271 (N117BF), intake valve failure, broken valve spring retainer, cylinder No. 2 o S/N 9569181 (N562TU), exhaust valve failure, broken valve spring retainer, cylinder No .1 All the engines had differing hours of operation; however, all experienced a valve spring retainer failure during engine operation. At